Phases Of A Cell Cycle Biology Essay

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Interphase is a part of the cell cycle in which the cell reproduces more DNA, it is the time when a cell accomplishes the most this is the time that the cell is preparing to divide. It is duplicating its organells and increasing the amount of cytoplasm it has in order to undergo cell division interphase constitutes the longest period of the cell cycle and is divided into three successive phases.

G1 phase

S phase

G2 phase

2.1.1. G1 Phase:

The first part of the inter phase is G1. In G1phase the cellular metabolic rate is high in which cell normally grows in size. And synthesis of m-RNA, t-RNA, r-RNA and proteins occur. This phase involves accumulation and synthesis of specific enzymes needed to control DNA-synthesis and production of DNA base unit so that the supply is at hand when synthesis being.

2.1.2. S Phase:

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The S phase is the intermediate phase between G1growth phase (gap-1) and G2 growth phase (gap-2).During the portion of cell cycle the DNA molecule are actually duplicated to produce two similar daughter cells.

2.1.3. G2 Phase:

During the gap between DNA synthesis and mitosis, the cell will continue to grow. It is also called pre-mitotic phase. In this phase energy is stored for chromosome movement. Mitotic specific protein RNA and microtubule subunit are synthesis.

2.2. Mitotic Phase:

This phase involves the division of nucleus and division of cytoplasm. At this stage the growth and protein production stop. All of the cell cycle focused on the complex and orderly division into two similar daughter cells. Mitosis is much shorter than inter phase. Lasting phase only one two hours. As in both G1 and G2, there is a checkpoint in the middle of mitosis that ensures the cell is ready to complete cell division actually stages of mitosis can be viewed at animal cell mitosis.

3. Mitosis:

It is a process of nuclear and cell division in which the numbers of chromosomes in daughter cells are same as that of parent cell.

4. Phases of Mitosis:

Mitosis is a continuous process which has two phases.

Karyokinesis

Cytokinesis

4.1. Karyokinesis:

This phase involves the nuclear division .Animal cell has a special structure for cell division called centriole. At the start of mitosis centriole is divided into two pairs. Each pairs migrate to the opposite side of the nucleus thus forming two poles of the dividing cell.

The microtubules including aster and spindle are called mitotic apparatus. There size is larger than nucleus. The function of the mitotic apparatus is to attach and capture chromosomes, align them correctly and separate them so that equal distribution of chromosomes should take place.

The microtubules are set of fibers which are three in number. They originate from each pair of centrioles. One set of microtubules is called astral microtubules which form aster outward. The other two sets form the spindle in which one is kinetochore microtubules and other is polar microtubules and other is polar microtubules.

4.1.1. Stages of Karyokinesis:

Karyokinesis can be divided into four stages.

Prophase

Metaphase

Anaphase

Telophase

4.1.1.1. Prophase:

Chromosomes can not be seen in the interphase stage even by electron microscope. However by using some special stains chromatin network of fine threads can be seen which is called chromatin. The following events take place in prophase.

Nuclear envelope along with nucleoli disappear

Centrioles completely migrate towards their poles

Mitotic apparatus is organized

Cytoplasm becomes more viscous

The chromatin material get condensed by folding and chromosomes appear as thin threads and eventually become thick consisting of two chromatids attached with centromere

4.1.1.2. Metaphase:

At metaphase chromosomes have become completely thick consisting of chromatid. These chromatids are attached to centromere. The Cells has special area the kinetochore where kinetochore fibres of mitotic apparatus attach. The kinetochore fibers of spindle are attached to kinetochore region of chromosomes and align them at the equator of the spindle forming equatorial plate or metaphase plate. Each kinetochore gets two fibers each from opposite poles.

4.1.1.3. Anaphase:

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It is very important phase of mitosis. The kinetochore fibers of spindle contract toward their respective poles exert force which separates the two chromatids from centromere. In this way two sister chromatids are formed. Half of them move to one pole and other half towards other pole.

4.1.1.4. Telophase:

As the chromosomes reach to their respective pole telophase starts. The chromosome starts unfolding and become chromatin material. Mitotic apparatus starts disappearing while nucleus and nuceolic appears and thus two nucleoli are formed at each pole of the cell.

4.2. Cytokinesis:

Cytokinesis is a phase of the mitosis in which whole of the cell divides. At this phase the astral microtubules send signals to equatorial region of cell which activate action and myosin to form contractile ring. This contractile rings form cleavage furrow in the center of cells which get deepens gradually and ultimately divide the parent cell into two daughter cells.

5. MEIOSIS:

Meiosis is a special type of cell division in which the numbers of chromosomes in daughter cells are reduced to half as compare to parent cell.

In animals it occurs in diploid cells at gamete formation while in plants it takes place at spore formation at gamete formation.

Meiosis can take place in two divisions

Meiosis I

Meiosis II

The first meiotic division is reduction division and second meiotic division is like that of mitosis. After two divisions diploid cell produce four haploid cells.

5.1. Stages of Meiosis I:

Meiosis I is further divided into four sub-stages as following

Prophase I

Metaphase I

Anaphase

Telophase

5.1.1. Prophase I:

Prophase I is a very long phase. The chromosomes in this phase behave like the homologous pairs which is different as that from mitosis. The similar chromosomes are called homologous chromosomes. Prophase I is divided into five sub stages.

Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis

5.1.1.1. Leptotene:

At this stage the chromosomes appear as very long narrow thread and the size of nucleus increase and homologous chromosomes become closer.

5.1.1.2. Zygotene:

First essential phenomenon of meiosis i.e. pairing of homologous chromosomes called synapsis starts. This pairing is highly specific and exactly pointed, but with no definite starting point. At this phase nuclei also disappears. Each paired but no fused complex structure is called bivalent or tetrad.

5.1.1.3. Pachytene:

The pairing of homologous chromosomes is completed. Chromosomes become more and more thick. Each bivalent has four chromatids, which wrap around each other. Non-sister chromatids of homologous chromosomes exchange their segments due to chiasmata formation. During the process called crossing over. In this way reshuffling of genetic materials occur, which produces recombination.

5.1.1.4. Diplotene:

The next stage of meiosis is diplotene where bivalents are distinct and contracted. During this phase the chiasmata of each bivalent undergoes terminalization that are the movement of two homologous chromosomes to the two ends. During the stage of prophase due to chromosome contraction they are very distinct as visible bivalent structure.

5.1.1.5. Diakinesis:

Next phase is diakinesis where the chiasmata are fully terminalized and the two chromosomes remain together though fully terminalized.

5.1.2. Metaphase I:

Nuclear membrane disorganize at the beginning of this phase. Spindle fibres originate and the kinetochore fibres attach to the kinetochore of the homologous chromosomes from each pole and arrange bivalent at the equator. The sister chromatids of individual chromosomes in bivalent behave as a unit.

5.1.3. Anaphase I:

In this succeeding anaphase that is Anaphase I, the homologous centromeres move towards opposite direction of the pole. The centromere of each chromosome remains intact. The chromosome being separated there is no chaismata at this stage.

5.1.4. Telophase:

Nuclear membranes organize around each set of chromosomes at two poles. Nucleoli reappear thus two nuclei each with half number of chromosome is formed, later on, cytoplasm divides thus terminating the first meiotic division.

6. Summary:

Each cell cycle has two phase- interphase and mitotic phase. Inter phase is highly metabolic phase and subdivided into G1, S, G2 phases. Dividing phase involves the division of nucleus and division of cytoplasm.

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Mitosis is an equational cell division occurs in somatic cells producing two cells of the same chromosomes number of parent cell. Karyokinesis occurs in four phase â€"prophase, anaphase, metaphase, telophase and is followed by cytokinesis. Meiosis is a reductional cell division occurs in germ cells producing four cells of half chromosomes number from that of parent cell, Nucleus divides twice in meiosis. Prophase I is of longer duration with substages leptotene, zygotene, pachytene. diplotene, diakinesis and is characterized by bivalent formation and crossing over through chaismata formation.